1. Field of the Invention
The present invention relates to an apparatus and a method for forming a color toner image on paper or transparent overhead projector film by electrophotography.
2. Discussion of the Related Art
It has been conventional practice to form an image with color toner on a transparent film by the electrophotographic method, and project the image on a screen by an overhead projector or the like, but there is a tendency for contour to occur because the image to be projected is dark and indistinct in the parts of low or medium density. The cause of this phenomenon is considered to be that the fuser can sufficiently flatten the surface of the toner image in parts of high density, but the surface of the toner image in the parts of low density retains its unevenness, and light scattering caused by the unevenness of the toner surface affects the projected image. However, the precise cause of this phenomenon is not as yet known.
When using transparent film, to prevent the scattering of transmitted light, a method for flattening a surface of a toner image by spraying it with lacquer or the like has been considered. Japanese Patent Application Unexamined Publication (laid - open) No. Sho. 63-80273 (1988) discloses a method for removing tiering and gapping among toner by changing the shape of the toner particles and fusing them together into a single body to form a toner image with sufficiently even surface.
Further, the conventional electrophotographic method produces high refraction of the incident light at the interface of the toner and the air, and therefore it can only provide a dark projected image with low chroma.
To solve the problem mentioned above, a method for fusing the toner at a temperature in which the toner melts completely is also disclosed in Japanese Patent Application Unexamined Publication (laid - open) No. Sho. 63-80273 (1988). Japanese Patent Application Unexamined Publication No. Sho. 60-52861 (1985) describes a method for processing the surface of a color toner image to make the value of the surface roughness RZ as prescribed by JIS B0601-1982 a 10-point roughness height of 0.8 .mu.m.
However, in the method for flattening a surface of a toner image by spraying with lacquer or the like, there is the problem that the toner forming the image dissolves in the solvent, and thus the projected image loses its sharpness, color puddles appear, or discoloration occurs in the part where no toner image is formed. In addition, this method is not very efficient.
On the other hand, in the method for flattening the surface of the toner layer by changing the shape of the toner particles and fusing them into a single body, it is necessary to make the temperature of the fuser higher when using transparent film than when using paper in order to melt the toner sufficiently, which causes the problem of delays occurring when the transfer medium is changed. Further, mechanically smoothing the surface of the fused toner image requires an additional abrading device.
Attempting to make the parts of the image with less toner, that is the medium-density portions of the image, sufficiently smooth by a method of roller fusing at a high temperature as described in Japanese Patent Application Unexamined Publication No. Sho. 63-80273 (1988), tends to produce offset in portions with a large amount of toner. For this reason, although the surface of the toner image in portions of high density can be made adequately smooth, in the portions of medium density unevenness remains, whereby the incident light is scattered, and contour occurs in the medium-density portions of the image. Thus contour sometimes occurs in the medium-density portions of the image.
In particular, because a digital color copying machine controls the density of each color by variation of the area of very fine dots or lines, unevenness of the surface of the toner image is caused by the dots and lines of the halftone portions of the image; accordingly contour on the toner surface in the halftone portions tends to occur in the process of making a high quality projected image for an overhead projector using a digital color copying machine.
The embodiment of the method described in Japanese Patent Application Unexamined Publication No. Sho. 60-52861 (1985) does not specify an upper limit of the accumulated depth of the toner image construction when unevenness in the image construction has a specific spatial frequency as in a digital color copying machine.